During tumor development and progression, cancer cells encounter cytotoxic conditions such as hypoxia, nutrient deprivation, and low pH due to inadequate vascularization (Hanahan, D., et al. 2011. Cell 144, 646-674). To maintain survival and growth in the face of these physiologic stressors, a set of adaptive response pathways are induced. One adaptive pathway well studied in other contexts is the unfolded protein response (UPR), which is induced by factors affecting the endoplasmic reticulum (ER) such as changes in glycosylation, redox status, glucose availability, calcium homeostasis or the accumulation of unfolded or misfolded proteins (Hetz, C., et al. 2001. Physiol Rev 91, 1219-1243). Notably, features of the tumor microenvironment, such as hypoxia and nutrient deprivation, can disrupt ER homeostasis by the perturbation of aerobic processes such as oligosaccharide modification, disulphide bond formation, isomerization, and protein quality control and export (Wouters, B. G., et al. 2008. Nat Rev Cancer 8, 851-864).
In mammalian cells, the UPR is mediated by three ER-localized transmembrane protein sensors: Inositol-requiring transmembrane kinase/endonuclease-1 (IRE1), PKR-like ER kinase (PERK) and activating transcription factor 6 (ATF6) (Walter, P., et al. 2011. Science 334, 1081-1086). Of these, IRE1 is the most evolutionarily conserved branch. An increase in the load of folding proteins in the ER activates IRE1, an ER-resident kinase and endoribonuclease that acts as an ER-stress sensor (Walter, P., et al. 2011. Science 334, 1081-1086). Activated IRE1 removes a 26 bp intron from XBP1 mRNA and results in a frame shift in the coding sequence, with the spliced form encoding a 226 amino acid transcriptional activation domain (Calfon, M., et al. 2002. Nature 415, 92-96; Yoshida, H., et al. 2001. Cell 107, 881-891). In contrast to the unspliced XBP1 (XBP1u), which is unstable and quickly degraded, spliced XBP1 (XBP1s) is stable and is a potent inducer of target genes that orchestrate the cellular response to ER stress (Hetz, C., et al. 2011. Physiol Rev 91, 1219-1243). XBP1 deficient mice display severe abnormalities in differentiation of several lineages of specialized secretory cells, including plasma cells (Reimold, A. M., et al. 2001. Nature 412, 300-307), exocrine pancreas cells (Lee, A. H., et al. 2005. EMBO J 24, 4368-4380) and intestinal epithelial cells (Kaser, A., et al. 2008. Cell 134, 743-756). As the mammary gland is a secretory tissue that undergoes extensive secretory compartment expansion during the transition from pregnancy to lactation, the function of XBP1 in the normal mammary gland and in breast cancer is of special interest. XBP1 expression was reported to be regulated by estrogen receptor and induced in primary human breast cancer (Fujimoto, T., et al. 2003. Breast Cancer 10, 301-306), however, the functional role of the UPR and XBP1 in the normal and malignant mammary gland is largely unknown.